A technique of electric power steering (EPS) control is known. In this technique, EPS control is performed on the basis of an output signal (motor torque signal) from a torque sensor attached to a rotating shaft of an assist motor that assists the steering effort of the driver.
Japanese Unexamined Patent Application Publication No. 11-5548 discloses an electric power steering apparatus that includes a speed reducing mechanism, in which a pinion gear mounted on a rotating shaft of an assist motor engages with a hypoid gear mounted on a steering shaft.
In this electric power steering apparatus, changes in the torque of the assist motor cause changes in backlash in the engaging part of the speed reducing mechanism (see, paragraph [0033] in Japanese Unexamined Patent Application Publication No. 11-5548).
Japanese Unexamined Patent Application Publication No. 11-5548 states that it is possible to compensate for the changes in backlash by varying the voltage applied to a piezoelectric collar that regulates the degree of engagement between the hypoid gear and the pinion gear, in accordance with changes in output signal (motor torque signal) from a torque sensor attached to the rotating shaft of the assist motor (see, paragraphs [0021], [0034] to [0037] in Japanese Unexamined Patent Application Publication No. 11-5548).
However, Japanese Unexamined Patent Application Publication No. 11-5548 does not specifically describe electrical and mechanical configurations of the torque sensor, and only states that the torque sensor is connected to an electronic control unit (ECU) (see, paragraph [0024] in Japanese Unexamined Patent Application Publication No. 11-5548).
Japanese Unexamined Patent Application Publication No. 2006-7931 discloses a technique in which the torque of an assist motor is controlled on the basis of a target motor torque signal and a torque sensor signal (actual motor torque signal) output from a magnetostrictive torque sensor attached to a rotating shaft of the assist motor (see, paragraphs [0018], [0019], [0022] to [0025] in Japanese Unexamined Patent Application Publication No. 2006-7931).
Japanese Unexamined Patent Application Publication No. 2009-264812 discloses a more detailed configuration of the magnetostrictive torque sensor.
Japanese Unexamined Patent Application Publication No. 2006-7931 does not specifically describe calculation and control techniques used by a motor current calculating unit that drives the assist motor equipped with the magnetostrictive torque sensor, but states that the assist motor includes a resolver serving as a rotational phase sensor (see, paragraph [0023] in Japanese Unexamined Patent Application Publication No. 2006-7931).
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-7931 is assumed to provide a control configuration in which a minor loop includes vector control (current control) of related art which involves using a high-resolution resolver with a complex configuration to detect a continuous rotational phase (angle) that increases in increments of one degree (1°) or the like, and a major loop includes torque control which is provided outside the current control to stabilize the motor output.
In this case, the assist motor has a complex structure, because it requires a magnetic path for generating a magnetic flux density that varies sinusoidally for vector control. The control circuit and the control algorithm are also complex.
Generating a magnetic flux density that sinusoidally varies allows sinusoidal energization (sinusoidal drive) of a 180-degree energization type, and can prevent changes in motor torque. However, generating a magnetic flux density that sinusoidally varies requires an advanced and complex operation for designing a complex stator teeth shape, cross-sectional shape of permanent magnets, and polarization direction. Additionally, for passing an energization current for sinusoidal energization, vector control needs to be performed by an expensive high-speed microcomputer on the basis of an angle detected by an expensive high-resolution resolver.